A review and analysis are presented of the literature on the fracture toughness of 9% nickel steels with emphasis on the crack arrest properties of these steels applicable to LNG storage vessels. The concept of crack arrest is examined and the merits of the various tests that have been developed to measure crack arrest capability are discussed. The gaps in present knowledge are pointed out and four research programs are suggested to remedy these gaps: ( 1 ) the significance of the weld heat-affected zone to crack arrest@ (2) correlation of crack initiation and crack arrest tests@ (3) characterization of the fracture toughness of current 9% nickel steels@ and (4) the effect of fabrication defects on crack initiation in 9% nickel steels. A bibliography is included.